The foamed insulation layers of foamed electric wires used in high-speed transmission cables such as USB 3.0 cables, HDMI cables, InfiniBand cables or micro USB cables are required to have a narrow diameter, have high heat resistance and enable the formation of fine foam.
Although polyethylene (PE), which demonstrates superior electrical characteristics at high frequencies, has typically been conventionally used for the insulating material that composes the foamed insulation layer, PE does not allow the formation of adequately fine foam in addition to being unable to satisfy the requirement of an upper limit of heat resistance of about 90° C. as required for the high-frequency cables described above. Consequently, it has been proposed to obtain a foamed insulation layer having superior heat resistance and foamability by using a propylene-based resin having prescribed characteristics (Patent Document 1).
Patent Document 1: Japanese Patent Application Laid-open No. 2005-78835